Natural gamma-ray elemental concentrations in formations provide information that is useful for petrophysical evaluation purposes. For example, thorium and potassium are useful for mineral identification, especially when combined with other logging measurements. As a further example, zones with high uranium concentrations can be used to identify volcanic zones, and in many organic shale reservoirs, uranium content often correlates with total organic carbon content.
Various methods have been developed to derive these elemental concentrations from pulse-height natural gamma-ray spectra. However, deviations from measurement instrument calibration settings, including those for spectrometers, can occur for a variety of reasons. For example, voltage drifts due to changes in temperature can lead to deviations in spectrometer gain and channel offset. Similarly, light output from most scintillation crystals varies with changes in crystal temperature which, in turn, affects the gain of detector systems. In addition, aging photomultiplier cathodes can cause spectrometer gain to change over time. For some detector systems a phenomenon known as photomultiplier fatigue occurs when large counting rate changes occur over short intervals of time. In well logging, spectrometers are often operated for long periods of time, and it is not uncommon for the gain to change because of some or all of these effects.
A variety of techniques have been developed in an attempt to compensate for departures from the desired energy scale calibration of gamma-ray spectroscopy measurements. Some conventional techniques involve continuously monitoring the location of a photopeak, requiring that one or more identifiable photopeaks be present in the observed spectra. Other techniques introduce a known source of gamma rays into the detector system to be superimposed on the spectrum produced by gamma rays external to the spectrometer. However, small inaccuracies in determining the known source's location in the spectrum can lead to gain adjustments that magnify errors at larger gamma ray energies. Further, using chemical sources as a calibration reference often creates a number of safety, security, regulatory, and environmental concerns.